Fix Windows system directory detection for anchored paths

[AperturePlus]

Motivation

• Property tests were failing on non-Windows CI because _is_windows_system_directory produced false negatives for Windows paths that include an anchor (for example C:\Program Files (x86)), due to how PureWindowsPath.parts represents anchored paths on POSIX hosts.

Description

• Update _is_windows_system_directory in src/aci/core/path_utils.py to inspect PureWindowsPath.anchor and strip Windows anchor prefixes (e.g. C:\, \\server\share\) before examining leading path components against WINDOWS_SYSTEM_DIRS.
• Preserve the defensive fallback that strips c:-style drive prefixes when present.
• Leave the existing resolved-Path fallback logic intact so native Windows behavior is unchanged.

Testing

• Ran pytest tests/unit/test_cli.py::TestCLIHelp::test_index_help tests/unit/test_cli.py::TestCLIHelp::test_search_help and both tests passed locally.
• Attempted to run the property tests that originally failed, but test collection aborted with ModuleNotFoundError: No module named 'hypothesis' in this environment, so the property-suite validation could not be completed here.
• Prior to the fix the full test run reported 4 failed, 665 passed; the change addresses the Windows-path detection failure that caused the property test failures in CI.

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Codex Task

[AperturePlus]